In addition to being pests, blood-feeding insects such as mosquitoes spread deadly infectious diseases including malaria, West Nile virus, yellow fever, and dengue fever between humans. Understanding the mechanisms by which mosquitoes are attracted to human hosts is therefore an important public health issue. One method of personal protection from mosquito bites is the synthetic molecule DEET (N,N-diethyl-meta-toluamide). Although it has been in use for more than 60 years, we still lack a complete understanding of the mechanism of action of this insect repellent. It is known that DEET repellency has at least two components: a volatile effect and a contact-mediated effect. Previous work has shown that the volatile effect of DEET requires the highly conserved insect olfactory co-receptor orco. However, the mechanisms of DEET contact chemorepellency are currently unknown. In Preliminary Studies, I showed that Aedes aegypti mosquitoes found DEET and other bitter substances unpalatable when mixed into sucrose solutions. However, the bitter taste of DEET does not account for its repellency because bitter substances applied to skin did not interfere with mosquito biting, while DEET prevented most bites. To determine which appendages sense DEET on contact, I carried out experiments in which only the proboscis had access to skin. Under these conditions, mosquitoes blood-fed on DEET-treated skin. This suggests that the proboscis is not sufficient to mediate DEET contact chemorepellency. To test if the legs are necessary for sensing DEET on contact, I occluded the hairs containing sensory cells on the legs with glue. Mosquitoes with glue-covered legs blood-fed on DEET-treated skin, indicating that the legs are required to sense DEET on contact. These Preliminary Studies lead to the Hypothesis that chemoreceptors in mosquito legs drive the contact repellency of DEET. The Objective of this proposal is to identify the DEET contact chemoreceptor(s). I propose two Specific Aims to test this Hypothesis and achieve this Objective. In Specific Aim 1, I will use bioinformatics to identify candidate genes expressed in the leg that may mediate DEET contact chemorepellency. In Specific Aim 2, I will generate and behaviorally characterize mosquito strains with targeted mutations in candidate DEET contact chemoreceptors. I hypothesize that mosquitoes with mutations in DEET contact chemoreceptor(s) will be less repelled by DEET, will spend more time on DEET-treated arms, and will bite DEET-treated arms more frequently. The research training plan will develop my skills in bioinformatics, genome modification, and behavioral analysis. The health- relatedness of this work is that mosquitoes and other biting arthropods spread deadly diseases to humans. A better understanding of insect chemosensation and mechanisms of repellency may lead to the design of better, longer-lasting arthropod repellents that will reduce the spread of infectious diseases.